This invention relates to electrodes and has particular reference to electrodes for use in electrochemical applications. An electrochemical application is one in which the electrode is inserted into an electrolyte and acts to conduct electrical current from the electrode into the electrolyte. In most cases the electrode would act as an anode.
Electrodes are well known in the form of a metal substrate of a film-forming metal, normally chosen from the group titanium and niobium, with an outer layer of an anodically active material which is normally a material containing a platinum group metal or a platinum group metal oxide. The platinum group metals or oxides may be used on their own or in conjunction with other materials which may be regarded as diluents or carriers.
There are many methods of applying the platinum group metals or metal oxides forming the anodically active layer to the film-forming metal substrate, some of which involve the application of heat to the coated substrate with the coated substrate being heated in an oxygen-containing atmosphere such as air. Other methods of application do not require heating in an oxygen-containing atmosphere. Such other methods include electroplating, metallurgical bonding by rolling or co-extrusion or application techniques which involve heating in a vacuum such as ion plating.
The present invention is particularly concerned with application methods which involve the heating of the anodically active layer either in its final form or in its compound form in an oxygen-containing atmosphere.
In British Patent Specification No. 1 274 242 there is described an electrode construction in which a substrate of titanium or niobium has bonded to it a metal foil chosen from the group tantalum and niobium (tantalum only in the case of a niobium substrate) with an outer layer of a platinum group metal foil. The outer platinum group metal foil is bonded directly to the substrate by local electrically generated heat. Such a prior specification does not describe the use of a painted and fired platinum group metal layer.
In European Patent Application Publication No. 0 052 986 there is described the use of an interlayer of an oxide of a metal chosen from the group titanium, tantalum, zirconium, hafnium and niobium in which the oxide layer is partially reduced to form a sub-oxide, which sub-oxide acts as an intermediate coating between the substrate titanium and the anodically active materials. Such a prior specification does not describe the coating of niobium nor does it describe the use of a metallic interlayer between the substrate and the anodically active layer.
By "anodically active" as is used herein is meant a material which will pass significant electrical current when connected as an anode without passivating or without dissolving to any significant extent. Such an anodically active layer is the basis of a dimensionally stable anode in which the anode passes a current without significantly changing during the passage of the current.